1. Field of the Invention
The present invention relates to a polygon mirror scanner motor, and in particular a polygon mirror scanner motor having a polygon mirror fitted to a rotor.
2. Description of the Related Art
A polygon mirror scanner motor is a motor which is mounted on a laser beam printer or the like so as to rotate a polygon mirror at a high speed. A laser diode emits a laser beam onto a polygon mirror rotationally driven by a polygon mirror scanner motor. A laser beam for scanning a surface of a photoreceptor is generated by reflecting the laser beam on the polygon mirror.
The polygon mirror typically has a fitting, hole formed in its center portion. The inner circumferential surface of the fitting hole is fitted to the outer circumferential surface of a rotor. Typically there is a clearance between the inner circumferential surface of the fitting hole and the outer circumferential surface of the rotor in order to make it easy to attach/remove the polygon mirror to/from the rotor. In a typical polygon mirror scanner motor, the polygon mirror is attached to the rotor by use of a holding spring, a screw, a bonding agent, etc.
For example, JP-A-11-183831 discloses a polygon mirror driving device including a rotary polygon mirror which includes a fitting hole, a rotor which includes a cylindrical portion fitted into the fitting hole of the rotary polygon mirror, and a mirror holding member which is screwed down to an upper portion of the cylindrical portion of the rotor. The mirror holding member includes a cap which is screwed down to the rotor, and a holding spring which is compressed between the cap and the rotary polygon mirror by a thrust-direction force applied from the cap. The rotary polygon mirror is attached to the rotor by a force applied from the holding spring and a bonding agent. The rotary polygon mirror and the mirror holding member are fitted to, the cylindrical portion of the rotor, parts having the same diameter, respectively.
JP-A-2000-347120 discloses a polygon mirror type light deflector including a rotor, a ring-like polygon mirror which has a center hole, a part of the rotor which it is inserted into and which is mounted on the rotor, and a cap which presses the polygon mirror to a mirror mounting portion and attaches the polygon mirror thereto. A mirror attaching portion of the cap is attached to the polygon mirror by a bonding agent. The polygon mirror and the cap are fitted to the portion of the rotor having same diameter.
Generally when a rotatable body is rotating, a centrifugal force proportional to the size of a radius from the center of the rotation axis of the rotating body acts on the rotating body so that the rotating body expands. As soon as the rotation stops, the centrifugal force disappears and the rotating body is restored to its original size. Thus, a rotating body that has a larger outer diameter, has a larger centrifugal force that acts on the rotating body so that the volume of expansion increases.
The polygon mirror scanner motor rotationally drives the polygon mirror at a high speed of about 40,000 rpm. The outer diameter of the polygon mirror is larger than the outer diameter of the rotor. Therefore, the centrifugal force acting on the polygon mirror is larger than the centrifugal force acting on the rotor. Thus, the clearance between the outer circumferential surface of the rotor and the inner circumferential surface of the fitting hole, which is small when the rotation stops, increases due to a difference between the centrifugal force acting on the polygon mirror and the centrifugal force acting on the rotor during the rotation. As a result, when the polygon mirror is rotated (started up) and stopped repeatedly, the posit on of the polygon mirror relative to the rotor gradually shifts, and this causes a submicron shift in the center of gravity of the polygon mirror, this problem causes deterioration of stability.